A transistor, which is a kind of a semiconductor device, includes a first insulation layer, a second insulation layer, an oxide semiconductor layer, and first to third conductive layers. The oxide semiconductor layer includes a region in contact with the first insulation layer, the first conductive layer is connected to the oxide semiconductor layer, and the second conductive layer is connected to the oxide semiconductor layer. The second insulation layer includes a region in contact with the oxide semiconductor layer, and the third conductive layer includes a region in contact with the second insulation layer. The oxide semiconductor layer includes first to third regions. The first region and the second region are separated from each other and the third region is located between the first region and the second region. The third region and the third conductive layer overlap with each other with the second insulation layer located therebetween. The first region and the second region include a region having a higher carbon concentration than the third region. An example of such a semiconductor device is disclosed in Japanese Unexamined Patent Application Publication No. 2015-181151.
In such a semiconductor device, the third conductive layer is used as a gate electrode layer and a conductive layer is included on an opposite side from the third conductive layer side with respect to the oxide semiconductor layer. The conductive layer is used as a second gate electrode layer (a back gate). According to such a configuration, a drain current is to be increased. However, the oxide semiconductor layer includes oxide semiconductor material as the semiconductor material and charge mobility is lower and the drain current is smaller compared to the configuration using polysilicon as the semiconductor material. Therefore, if using the oxide semiconductor material instead of polysilicon, the drain current is required to be further increased.